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In order to develop a stimulating yet effective school-based program which elicits
a positive bone growth response, we need to understand the forces acting on the bones of
children during various high-impact activities. One activity found to promote bone
growth is drop landings from a height. We determined whether there are other jumping
activities that exhibit similar loading properties to those of a drop landing and identified
the effects of number of feet used, continuity, and direction on loading.
Twenty-one healthy children (11 boys, 10 girls; age 7-9 years) were recruited from
the local population. After warming up, each child performed five trials of 13 types of
jumps as motion capture and ground reaction force data were collected. One type of
jump was a drop landing from a 61 cm-high platform. The other 12 types were performed
from the ground and comprised all possible combinations of three factors: direction
(vertical as high as possible, forward a distance of 80% body height, sideways a distance
of 55% body height), feet used (1-footed hops, 2-footed jumps), and continuity (discrete,
continuous). The average peak force and peak loading rate at the hip of the dominant
limb over five trials were computed and normalized to body weight for analysis. Three-way
ANOVA identified loading differences across direction, feet used, and continuity
among the 12 jump types. Paired t-tests with a Bonferroni correction compared the
loading for each activity to that of the drop landings.
In general, peak forces and loading rates during landing were greater for hops than
for jumps, greater for discrete than for continuous hops/jumps, and greater for forward
than for vertical hops/jumps. However, peak forces did not differ between sideways hops
and jumps, nor did peak loading rates differ between vertical hops and jumps. Peak
forces during the drop landings exceeded those during all other jumping activities except
the discrete forward hop, which had peak forces similar to the drop landing. Although
discrete forward hops had greater peak loading rates than the other 11 activities from the
ground, the rates were less than those during the drop landings. Likely, the need to arrest
the body's large forward momentum using a single limb upon landing of the discrete
forward hop elevated loading over the other 11 conditions and made it comparable to that
of the drop landing.
Knowing what factors influence impact forces and loading rates on the hip,
specifically that discrete forward hops have high forces similar to those for drop landings,
will aid in developing a stimulating and effective jumping program for improved bone
development in children.